SW doesn't accept hyperbolic functions like cosh in equations or equation driven curves. Try the longer form:
0.5*a*(exp(t/a) + exp(-1*(t/a)))
I agree that SW doesn't like COSH(a), but I just want to point out that the help file says that SW does do hyperbolic cosine. See the link.
Maybe SW programmers need to talk to SW help file writers. I cannot make this work for an equation derived function either.